1. Field of the Invention
This invention relates to a surface-mounted-type inductance element such as a pulse transformer or the like which is assembled in a hybrid integrated circuit for a telecommunication equipment or a control device.
2. Description of the Prior Art
Referring now to FIG. 1, a conventional inductance element of this type will be described in order to facilitate understanding of the present invention. In FIG. 1, the conventional inductance element comprises a bobbin 1 having wires 2 coiled therearound and a pair of bases 3 integrally formed at opposing ends of the bobbin 1 in a manner to project laterally from the bobbin 1, from each of which bases 3 a plurality of external terminals 4 are projected outward, around which external terminals 4 terminations (not shown) of the wires 2 coiled around the bobbin 1 are wired; and a pair of cores 5 assembled to the bobbin 1. In the conventional inductance element, a coil portion is exposed to the external air, so that when the inductance element is mounted on a printed circuit board by, for example, reflow-soldering the external terminals 4 to conductive patterns which are previously formed on the printed circuit board, heat required in the reflow-soldering will badly affect the coil portion. In addition, cleaning of the inductance element mounted on the printed circuit board is generally carried out by using a solvent, so that the solvent will also badly affect the coil portion. Furthermore, since the coil portion is exposed to the external air as described above, the conventional inductance element is unable to resist moisture and is considerably susceptible to an external environment.
It is disclosed in Japanese Utility Model Registration No. 1,305,150 (Japanese Utility Model Publication No. 7320/1979) to TDK Corporation that, in order to overcome the disadvantages of the above-described conventional inductance element, the whole inductance element including a core is covered with plastic materials, with terminations of a wire coiled around the core being projected outward. However, in this case, when the whole essential parts of the inductance element including the core are covered with plastic materials by molding, stress associated with hardening of the plastic materials and stress associated with expansion and contraction of the plastic materials occuring due to a temperature change may degrade characteristics of the core. Concerning a low impedance electronic or electric component which constitutes an open magnetic circuit, such degradation of the characteristics may be tolerated to a certain extent, but, in regard to an electronic or electric component which is to constitute a closed magnetic circuit, the stresses as described above may cause cores to be separated from each other or destroy the cores, resulting in the magnetic characteristics of the electronic or electric component significantly deteriorating.